1. Field of the Invention
The present invention relates to a moving picture coding device that predicatively codes a moving picture.
2. Description of the Related Art
Moving picture data (dynamic image data) generally has a large data size and is therefore coded at a high efficiency (high-efficiency coding) when transmitted to a receiving device from a transmitting device or when stored in a storage device. The high-efficiency coding is a coding technology of converting a certain data train into one other data train and is a technology of compressing a target data size.
MPEG (Moving Picture Experts Group) exists as the standards for the moving picture coding. The MPEG involves using three predictive coding types. Hence, an MPEG frame is structure of three types of pictures corresponding to this predictive coding type. FIG. 8 is a diagram showing a fame structure in the MPEG. As shown in FIG. 8, the MPEG frame is structured of an I-picture (Intra coded picture) (I0 in FIG. 8), a P-picture (Predictive coded picture) (P3 and P6 in FIG. 8) and a B-picture (Bidirectionally coded picture) (B1, B2, B4 and B5 in FIG. 8).
The I-picture is a fame serving as a starting point of processing and is coded by intra-frame prediction. The P-picture is coded by unidirectional motion-compensated inter-frame prediction from the I-picture or the P-picture in the past. The B-picture is coded by bidirectional motion-compensated inter-frame prediction from the I-picture or the P-picture in the past and from an I-picture or a P-picture in the future. This motion-compensated inter-frame prediction method is a coding method that utilizes such a general property of the moving picture as to have a high similarity between the frame data at a certain timing and the frame data at a next timing in the frames configuring the moving picture.
Given further as the moving picture coding method using such moving picture high-efficiency coding is a method of at first predicting the generated code quantity (data size) acquired as a result of the coding that employs a predetermined coding scheme (coding parameters etc) and statistic data representing a property of the picture, next readjusting the coding scheme on the basis of the generated code quantity and the statistic data that are thus predicted, and actually coding the target frame. This method is called a 2-path coding method in some cases.
On the other hand, according to the coding method using a single-system encoder without employing the 2-path coding system, in the case of coding the moving picture in real time, there is not generally obtained information showing how much the coding quantity occurs as a result of coding the picture at a starting time of coding the predetermined picture. Hence, the coding method using the single-system encoder has difficulty of controlling a code quantity for adjusting a designated bit rate while taking affection on a picture quality into consideration, with the result that a bias in picture quality occurs within the picture.
In the moving picture coding device using the single-system encoder, a method of yielding an effect equivalent to the 2-path coding system is proposed in order to solve such a problem (Patent document 1 given below). In this coding method, to begin with, there is generated a reduced picture with a decreased resolution of the picture that should be originally coded, and the reduced picture is coded. Then, a coding parameter is adjusted based on a coded result, the originally-should-be-coded picture accumulated in a delay buffer is coded by the adjusted coding parameter, whereby the effect equivalent to the 2-path coding is acquired by the single encoder.
Note that the following disclosed documents are given as documents of the conventional arts related to the present invention of the application in addition to those described above. The conventional art documents are “Japanese Patent Application Laid-Open Publication No. 2002-16912”, “Japanese Patent Application Laid-Open Publication No. 11-298904”, and “Japanese Patent Application Laid-Open Publication No. 2001-298734”.
In the conventional arts described above, however, precedence coding is carried out by use of the reduced picture, then a real unreduced picture is coded by utilizing a result of this precedence coding, and therefore, in addition to actual coding time of the real image, coding time of the reduced picture needs being processed within 1-picture time.
Accordingly, the time expendable for coding the should-be-coded real picture is shorter than the 1-frame time, so that in the technique employing the conventional art described above, processing performance of the single-system encoder is required to be set higher than the performance of a normal encoder.
Moreover, in the moving picture coding system using the motion-compensated inter-frame prediction, with respect to the picture such as B-picture in which bidirectional motion-compensated inter-frame prediction is conducted, a picture frame in the future needs coding earlier than this picture frame, and hence an actually inputted frame sequence is required to be reordered in the sequence for coding (a reordering process). FIG. 9 illustrates an outline of the reordering process for the inputted picture frames. FIG. 9 is a diagram showing a conventional coding method that employs the reordering.
In such a coding method, however, a time lag (idling time in FIG. 9) occurs due to reordering for a period from an input of the first frames (B0 and B1) to a start of the first coding process (coding of I2). During a period of this idling time, the encoder executing the coding process comes to an idling status where the encoder performs none of the processes.
Similarly, the moving picture coding system has a method of coding the frames by such a drop-frame technique that the inputted pictures are thinned out (see FIG. 10). In such a coding method also, the encoder is in the idling status where none of the processes are executed for the frame time when the frames are dropped.
Thus, in the conventional arts, even when enhancing the processing performance of the encoder in a way that expands a hardware scale in order to actualize the high-efficiency coding equivalent to the 2-path coding system, there was a case where the performance of the encoder was not sufficiently utilized depending on the coding method.